In the aerospace industry, the use of fiber-reinforced polymer composites in primary and secondary structures of aircraft is becoming more prevalent. Composite structures are traditionally made by laying up plural layers (or plies) of resin-impregnated fibrous reinforcement (known as prepregs) on a mold surface, followed by consolidating and/or curing. The advantages of fiber-reinforced polymer composites include high strength-to-weight ratio, excellent fatigue endurance, corrosion resistance and flexibility, allowing for a significant reduction in component parts, and reducing the need for fasteners and joints. However, the application of these materials for modern aircraft's primary and secondary structures presents special challenges due to the dielectric nature of the matrix resin. Although the use of carbon fibers as reinforcing fibers in composite materials can deliver some degree of electrical conductivity along their longitudinal direction due to their graphitic nature, the dielectric properties of the matrix resins in the composite materials reduce the overall electrical conductivity of the composite materials.
Increasing the electrical conductivity of fiber-reinforced polymer composites is desirable in order to meet the requirements for lightning strike protection of an aircraft and to avoid a phenomenon called “edge glow” which is particularly critical for the composite wing assembly. The edge glow phenomenon manifests itself as a bright glow or spark in composite skin/spar assembly with energy sufficient to be a potential ignition source of fuel vapors.
This edge glow phenomenon may appear during a lighting strike event, especially on composite laminates having low z-direction electrical conductivity. During a lightning strike event, a transient charge with high intensity current travels through the skin and then enters the wing substructure (e.g. structural spar or ribs) because of the fasteners connecting the two composite parts. So typically, in a composite skin/spar assembly, current travels partially on the skin and partially through the spar which represents one of the walls of the fuel tank.
The current passes laterally from the fasteners through adjacent composite plies of the spar and tends to travel along the fibers because of the higher electrical conductivity as compared to the resin matrix. This path may generate the typical bright glow or sparks at the spar/rib cap edge, which is called “edge glow” phenomenon by those skilled in the art.
FIG. 1 shows a potential critical current path during a lightning strike event on a composite wing box. The edge glow phenomenon appears more critical when the resin between plies of fiber reinforcement is highly resistive, and consequently, the current tends not to flow between adjacent plies. If the z-direction conductivity is too low, significant voltage drops can be produced between plies during the strike, thus increasing the risk of edge glow.
As known by those skilled in the art, edge glow phenomenon is associated with electron surface ejections or plasma generation at the composite edges and often appears as a kind of resin explosion. Uncertainty regarding the nature of this phenomenon has posed several attentions in relation to the ignition capabilities of fuel vapors during a lightning strike event.
A conventional solution is to apply a sealant at the fuel tank (see FIG. 2). An example of such fuel tank sealant is the PR 1776 Class B sealant from LE JOINT FRANCAIS. However, such method leads to additional weight and is not always effective due to the lack of standardization and difficulties in the sealant application. Over time, the sealant becomes ineffective due to aging, or can be totally washed off by the fuel in the tank. Moreover, a lightning strike can result in the generation of high pressure gasses at the cut edge which may shatter the edge seal. There remains a need for a multifunctional composite material that can address the edge glow issue discussed above while providing good mechanical properties such as resistance to impact and delamination.